From 5fc960380ea44ba529c78b558b6cd4250e5e1958 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Sat, 22 Aug 2015 09:52:06 +0200 Subject: [PATCH] x86/fpu/math-emu: Fix math-emu boot crash On a math-emu bootup the following crash occurs: Initializing CPU#0 ------------[ cut here ]------------ kernel BUG at arch/x86/kernel/traps.c:779! invalid opcode: 0000 [#1] SMP [...] EIP is at do_device_not_available+0xe/0x70 [...] Call Trace: [] error_code+0x5a/0x60 [] ? math_error+0x140/0x140 [] ? fpu__init_cpu+0x59/0xa0 [] cpu_init+0x202/0x330 [] ? __native_set_fixmap+0x1f/0x30 [] trap_init+0x305/0x346 [] start_kernel+0x1a5/0x35d [] i386_start_kernel+0x82/0x86 The reason is that in the following commit: b1276c48e91b ("x86/fpu: Initialize fpregs in fpu__init_cpu_generic()") I failed to consider math-emu's limitation that it cannot execute the FNINIT instruction in kernel mode. The long term fix might be to allow math-emu to execute (certain) kernel mode FPU instructions, but for now apply the safe (albeit somewhat ugly) fix: initialize the emulation state explicitly without trapping out to the FPU emulator. Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Dave Hansen Cc: Denys Vlasenko Cc: Fenghua Yu Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Oleg Nesterov Cc: Peter Zijlstra Cc: Quentin Casasnovas Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar --- arch/x86/kernel/fpu/init.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c index 1e173f6285c7..d14e9ac3235a 100644 --- a/arch/x86/kernel/fpu/init.c +++ b/arch/x86/kernel/fpu/init.c @@ -40,7 +40,12 @@ static void fpu__init_cpu_generic(void) write_cr0(cr0); /* Flush out any pending x87 state: */ - asm volatile ("fninit"); +#ifdef CONFIG_MATH_EMULATION + if (!cpu_has_fpu) + fpstate_init_soft(¤t->thread.fpu.state.soft); + else +#endif + asm volatile ("fninit"); } /*